Method and apparatus for maintaining registration in a multistation printing press

ABSTRACT

In a web-fed printing press having a plurality of stations, each for printing one of a plurality of colors, registration is maintained by controlling the tension or elongation of the web between successive stations, in response to signals derived from a plurality of sensors, such signals being representative of registration error and the measured tension of the web.

United States Patent 1191 Roch 1451 Apr. 23, 1974 METHOD AND APPARATUS FOR MAINTAINING REGISTRATION IN A MULTISTATION PRINTING PRESS [75] Inventor: Roger-Henri Roch, Ecublens,

Switzerland [73] Assignee: J. Bobst & Fils S.A., Prilly,

Switzerland [22] Filed: Oct. 10, 1972 [21] Appl. No.: 295,969

[30] Foreign Application Priority Data Oct. 8, 1971 Switzerland 14749/71 [52] U.S. Cl. 226/2, 226/28 [51] Int. Cl B65h 23/18 [58] Field of Search 226/28-31,

226/113, 44; 101/228, DIG. 21, 248

[56] References Cited UNITED STATES PATENTS 2,840,372 6/1958 Alhand 226/28 3,031,118 4/1962 Frommer.... 226/28 3,525,858 8/1970 Thiede 226/28 X 3,667,664 6/1972 Schroeder 226/44 Primary Examiner- -Richard A. Schacher Attorney, Agent, or Firm-+1111, Sherman, Meroni, Gross & Simpson 5 7] ABSTRACT In a web-fed printing press having a plurality of stations, each for printing one of a plurality of colors, registration is maintained by controllingthe tension or elongation of the web between successive stations, in response to signals derived from a plurality of sensors, such signals being representative of registration error and the measured tension of the web.

11 Claims, 12 Drawing Figures PATENTEUAPR 23 m4 3 8 06; 0 '1 2 SHEET 3 UF 5 FIG. 6

METHOD AND APPARATUS FOR MAINTAINING REGISTRATION IN A MULTISTATION PRINTING PRESS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for maintaining register in a web-fed printing press having multiple stations.

2. Prior Art Multistation printing presses are commonly used for color printing, and it is necessary to provide some means of adjusting the position of the image printed by one station relative to thatprinted by another station, in order to correct for and eliminate misregistration of the various colors. A number of systems for making such adjustments have been developed in the prior art. In one system, a compensating roller is placed between two successive stations, and means is provided for moving the position of the compensating roller, in response to a detected misregistration, to effect a change in the length of the web extending between the two stations. The misregistration is corrected when necessary by providing a greater or lesser length of web between the stations, with the result that the image printed by the second station is shifted longitudinally relative to the image printed by the first station.

In other known systems, the angular displacement of the printing cylinder at one or the other of two successive printing stations is changed to effect a desired correction. When this is to be performed automatically, a sensor is employed to detect the magnitude and direction of the change which is needed.

While the systems of the prior art have been effective in their purpose of correcting misregistration, the operation has been relatively slow, since only the magnitude and direction of misregistration have been taken into account in the derivation of a control function which restores registration to normal, and the rate at which misregistration is being corrected has been ignored. Because of the length of web between the two stations, and the length of web between .the second station and the misregistration sensor, a time lag exists between the making of a corrective adjustment, and the sensors recognition of the adjustment. In order to avoid instability, the response time of the corrective system is extremely slow, and as the amount of misregistration decreases, the rate at which it decreases changes exponentially so that correct registration is approached asymptotically, and reached only a relatively long time after the misregistration is initially detected.

Therefore a method and apparatus for effecting a more rapid correction of misregistration, is extremely desirable.

SUMMARY OF THE INVENTION It is a principal object of the present invention to provide a method and apparatus for effecting a rapid correction of misregistration in a multistation web-fed press.

Another object of the present invention is to provide means for effecting the most rapid correction of misregistration possible, while avoiding breakage of the web by maintaining the web tension under a critical limit.

A further object of the present invention is to provide a method and apparatus for automatically maintaining the tension of the web at a value which insures proper registration.

Another object of the present invention is to provide a method and apparatus for employing the rate of correction as a factor in deriving a control function which restores registration to normal.

These and other objects and advantages of the present invention will become manifest upon a consideration of the following description on the accompanying drawings.

In one embodiment of the present invention there is provided, in a multistation web-fed press, tension responsive sensor means disposed between two successive stations for producing a signal in response to the tension of the web, misregistration sensor means disposed downstream of the second station for producing a signal responsive to the magnitude and direction of a registration error, and means for controlling the tension of the web between the two stations in response to a signal derived from said tension responsive sensor means and from said misregistration sensor means.

The present invention makes use of the fact that the error in registration which is measured downstream from the second station is a result not only of the relative orientation of the printing cylinders at the two stations, but also of the tension of the web extending between the two stations. Accordingly, suitable control of the tension of the web between the stations permits an optimal correction of the registration error. The appropriate tension for the web is calculated from the registration error detected following the second station, and when the tension of the web is held at the appropriate value, the registration error is rapidly brought to zero. The appropriate tension for the web is calculated by a computer which develops a signal in response to the existing tension on the web and the magnitude and direction of the existing misregistration error.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to the accompanying drawings in which:

FIG. 1 is a diagrammatic illustration of a press having two separate printing stations;

FIG. 2 is a functional block diagram of apparatus constructed in accordance with the invention for correcting the tension of the web between two stations of the press;

FIG. 3 is a functional block diagram of a computer employed in connection with the invention;

FIG. 4 is a schematic diagram of an illustrative embodiment of the present invention, employed with a press having two printing stations;

FIG. 5 is a functional block diagram of an alternative embodiment of the present invention;

FIGS. 6 and 7 both illustrate apparatus for imposing a predetermined tension on the web;

FIG. 8 is a functional block diagram of apparatus for deriving the tension of the web from the rate of change in registration error;

FIG. 9 is a diagramatic illustration of apparatus for generating a signal in response to web tension; and

FIGS. 10, 11, and 12 are a series of graphs demonstrating the correction of an initial registration error e DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I of the drawings, y represents the upward displacement, from a nominal position 0, of an idler roller 20 which roller supports the web 21 between two successive stations 22 and 23 of a multistation press. The actual tension of the web between the two stations is T, which is greater than the normal web tension T because of the displacement of the roller 20. The value of T of course depends upon the particular press, and also on the physical characteristics of the web 21 As illustrated in the graphs 24, 25 and 26 of FIG. 1, which respectively relate the registration error e, the displacement y and the web tension T to the length x of the web 21 passing through the two stations 22 and 23, the registration error is initially e the displacement is initially zero, and the tension of the web is initially T The most rapid correction possible is illustrated by the solid line curves in the graphs 24, 25 and 26 (FIGS. 12). The tension of the web is raised quickly to a relatively high value T by raising the position of the idler roller by an amount y. As a result, the web is elongated by a length equal to 2y and the registration error is decreased uniformly at a rate proportional to the elongation of the web. The linear decrease in the registration error e is illustrated in the graph 24 of FIG. 10,,and the increase in the tension of the web from T to T and the maintenance of the tension T is illustrated in the graph 26 of FIG. 12. The value T is ideally just below the breaking point of the web, to maximize the rate at which correct registration is approached.

As the web travels through the press, the idler roller 20 gradually changes its position so as to maintain the tension of the web at the desired value T. The displacement y of the idler roller is increased uniformly as illustrated in the graph 25 of FIG. 11.

When the registration error e has been reduced to 0, the tension of the web is immediately returned to the normal tension T by returning the idler roller to a position which gives that tension. Thereafter the tension on the web is maintained at its rated value T and registration is maintained.

If the registration error should subsequently increase to some value, the process is repeated as described above. The dashed lines included in the graphs 24, 25 and 26 illustrate, for comparative purposes, the effect of moving the idler roller 20 immediately to its final position to make a correction in registration error, as shown in the graph 25. From the graph 24, it is apparent that the correction is asymptotic and therefore takes a considerable time to accomplish, as compared with the more rapid linear correction function of the ideal case, illustrated in solid lines in the graphs 24, 25 and 26. The present invention, in controlling the tension of the web, approaches the ideal case much more closely than the systems of the prior art.

Referring now to FIG. 2, a computer l is provided in the present invention for the purpose of converting data, sensed from the condition of the web 21, into a signal representative of the computed tension T which it is the calculated optimum tension of the web between the successive stations, in order to bring about the corrective action described above. The signal generated by the computer 1 is a control signal and is connected to the input of a regulator 2. The regulator 2 receives a second input a signal T,,,, which is proportional to the measured tension of the web between the two successive stages of the press. The regulator 2 functions to produce at its output a signal in response to the difference between T,, and T and furnishes such signal to an actuator 3, which is the motive means which moves the position of the idler roller 20. The actuator 3 may conveniently be a solenoid, or a servo motor, both of which are well known. The movement of the idler roller 20 is in the direction which increasesthe existing tension of the web 21, if the quantity T,, is initially less than quantity T in order to bring T into identity with T Similarly, the web tension is reduced, by moving the idler roller downwardly (as seen in FIG. 1) if the quantity T is greater than T The computer 1 is illustrated in more detail in FIG. 3, which is a functional block diagram showing the various elements of the computer 1. The input of the computer l is a signal representative of the registration error e and this is used to calculate three separate parameters which are added together in a summing amplifier 8. The first parameter, which is derived in the block 4, is the error e, multiplied by a constant, K,,. The second parameter, derived in the block 5, is the sum of the errors, beginning at some arbitrary point, multiplied by a different constant K,. The parameter produced by the block 5 is derived by adding together the registration errors developed at successive sensings of the registration error, which sensings occur in regular succession as registration marks pass under a sensing head or misregistration sensor, as well known in the art. The third parameter, derived in the block 6, is equal to the difference between two successive registration errors, multiplied by a constant I(,,.

The parameter derived in the block 5 is limited in the block 7 to establish a maximum for the effect that the sum parameter may have on the subsequent calculations.

All three parameters are summed in the summing amplifier 8 and then passed through a limiter 9 which limits the maximum signal which can be passed to the actuator 3, in order to establish upper and lower limits on the tension of the web 21.

One additional input to the summing amplifier 8 is I representative of the integer l so that when the misregistration error is O (as well as the parameters derived in the blocks 5 and 6), the output of the summing amplifier is unity.

The output of the summing amplifier 8 corresponds to the ratio of the tension T, which is needed to correct misregistration and the nominal tension T This output is furnished to one input of a multiplier 10, which receives as another input a signal representative of the nominal web tension T which is derived by means of a device 11. The device 11 is preferably manually adjustable, so that a correct nominal tension may be selected by the operator for any particular web material which may be used. The product generated by the multiplier 10 is the required signal T The function of the limiter 9 is to limit the maximum amount by which the web tension may be changed from nominal, in order to avoid breaking the web, and to maintain sufficient tension for proper operation of the printing press.

Referring now to FIG. 4, the misregistration sensor 12 is shown at its location downstream from the second station 23. It develops the error signal e The sensor 12 is connected with the computer 1, which has been described in connection with FIG. 3.

The sensor 13 produces a signal in response the tension of the web between the two stages 22 and 23 of the press, and develops the T, signal which is furnished to the regulator 2. The computer 1 and the regulator 2, as well as the actuator 3, are connected as described in connection with FIG. 2, and function to control the position of the idler roller 20.

Although only two stations of the multistation press are illustrated in FIG. 4, it is evident that a separate idler roller may be associated with each two successive stations of the press, and the equipment illustrated in FIG. 4 is then repeated for each pair of stations, so that all registration errors are avoided.

Referring now to FIG. 5, there is shown, in functional block diagram form an alternative embodiment of the present invention. The construction and operation of the apparatus of FIG. 5 is the same as that which has been discussed above in connection with FIG. 3 except that the integer 1 input to the summing amplifier 8 is omitted, and the block 5, which derives the sum parameter, is provided with an additional input which is connected from the output of a differential amplifier l4. The'amplifier 14 performs the same function in the apparatus of FIG. 5 as the regulator 2 performs in the apparatus of FIG. 2, namely producing a signal representative of the difference between two input signals. The function of the separate input to the block 5 in the apparatus of FIG. 5 is to initially set the value of the quantity represented by the output from the block 5 equal to the output of the differential amplifier 14. This is accomplished by initially operating the single pole double throw switch 27 to connect the output of the differential amplifier 14 to the line 28, via the terminal Ma. This position of the switch 27 is referred to as the manual position, and it functions to cause the block 5 to produce an output corresponding to the quantity represented by the signal T,, produced by the sensor 13. This is so because the initial output of the summing amplifier 8, during start up, is zero, with the result that the output of 'the differential amplifier 14 is equal in magnitude to T,,,. With the output of the block 5 thus set, the output of the differential amplifier 14 drops to zero. The switch 27 is then moved to its other position, in which the output of the amplifier 14 is connected to the actuator or motor 3. This position of the switch 27 is the automatic position, because the operation of the apparatus automatically controls the web tension. With the apparatus of FIG. 5, the nominal tension T is not required for proper operation, because the calculated tension T is computed directly by the summing amplifier 8, rather than the tension ratio which is computed in the apparatus of FIG. 3. It is necessary, however, to modify in this event the values of the constants K K',, and K',, which are employed in deriving the outputs of the blocks 4, 5 and 6 of FIG. 5.

By the use of the apparatus of FIG. 5, the initial value of the calculated tension T is equal to the actual tension T,, at that time, and so sudden changes in the tension of the web are avoided.

In a typical case for the computer of FIG. 3, the nominal tension of the web is 50 Kg, and the tension controlling apparatus functions to increase or decrease the actual tension 50 percent of nominal, to a value between Kg. and 75 Kg. The constants for the computer of FIG. 3, and the limit values for the limiters 7 and 9 are:

KP 80 K1 20 K 40 limiter 7 0.5 to 0.5 limiter 9 +0.5 to 1.5

For the same case, when the apparatus of FIG. 5 is employed,

KID 20 limiter 7 I 25 to Kg. mi 9 25 to 75 Kg.

Preferably the computers of FIGS. 3 and 5 are analog in nature, and update their outputs approximately 5 times per second, the rate at which registration marks printed by the press pass under the misregistration sensor 12 when such marks are printed one per meter, and the web is moving at a velocity of about 300 meters per minute. An analog representation of the value of the registration error is placed in storage at the time of passage of each registration mark, and the necessary parameters are produced therefrom. Except for the sum, which is accumulated to continuously represent the parameter produced by the block 5, it is necessary to store only the last previous registration error. The 'computation may obviously be carried out by digital means, instead of by analog means, if desired.

In certain applications, and particularly in the case of webs formed of material which is easily stretched such as webs of synthetic material, it is difficult to measure the tension of the web directly by means of the sensor 13. In this case it is preferable to measure the tension inferentially, by measuring auxiliary parameters which depend on the tension, or to impose a calculated tension on the web by means of an open loop system, in which case the tension responsive sensor'13 is not required.

When an inferential measurement of the tension of the web is to be made, it is preferable to employ an online computer in connection with the press, which calculates a value for the computed tension T on the basis of an imperically'derived relationship and controls the position of the idler roller 20 accordingly.

When the tension is to bemaintained at the calculated value by means of an open loop system, an apparatus which may be called a tension source" is employed. Although such a tension source may be formed in a variety of ways, one preferred construction of such a tension source comprises a pair of rollers 29 and 30 (FIG. 6) for causing the web to form a loop 31 therebetween, with a roller 32 forced toward the end of the loop by a piston in a pneumatic cylinder 33. The web tension is then proportional to the air pressure within the cylinder 33, which is in turn controlled by the output of the regulator 2 or the differential amplifier 14.

An alternative tension source is shown in FIG. 7, where the web 21 is caused to pass around a roller 34 having a shaft 35. The shaft 35 is supported by a lever arm 36, which in turn is supported at a fulcrum 37. A

means of a motor 39, which responds to the output of the differential amplifier 14, which receives one input from a sensor 40 in response to the position of the weight 38. The tension imposed on the web 21 is proportional to the distance of the weight from a balance point on the arm 36 so the output of the sensor 40 is proportional to the tension imposed on the web 21.

One means for determining the web tension inferentially from a change in the registration error is illustrated in FIG. 8. In FIG. 8, two successive stations of a printing press are illustrated, with two sensors 41 and 42 disposed at positions following each station, respectively. A third sensor 43 senses the vertical position of the idler roller 20, which is controlled by the motor 3, in the same manner which has been described.

Inputs from the three sensors 41,42 and 43 are connected to a computer block 44, which calculates a function for the measured tension from the three input signals. The sensor 43 is also connected to the computer l, which has been described in connection with FIG. 2, and the regulator or differential amplifier 2 energizes the motor 3 in accordance with the difference between the measured tension, represented by the output of the block 44, and the calculated tension represented by the output from the, computer 1.

The block 44 derives a value .which is proportional to the actual tension of the web 21 by making use of the fact that the tension of the web is directly proportional to its elongation. The elongation of the web is calculated by calculating the rate of change of the misregistration error (by subtracting the current value of the error from the previous value, as in the block 6 of FIG. 2) at the location of each of the sensors 41 and 42 in blocks 45 and 46, respectively, and subtracting one from the other in the block 47, to give a signal representative of the net increase (or decrease) in the quantity of the web which has entered the area between the two stations 22 and 23 (or departed from that area) within the period between two successive registration marks. This quantity is accumulated, by the block 48, so that the output of the block 48 is proportional to the total change, from some initial value, of the quantity of the web present between the two stations 22 and 23. This quantity is subtracted, in the block 50, from the output of the block 49, which is representative of twice the change, during the preceding period between two registration marks, in the height of the roller 20. The output of the block 50 therefore gives the change in elongation of the web between the stations 22 and 23 during the period between two registration marks. This quantity is added (or subtracted if the quantity is negative) in the block 51 to the total of the changes in elongation to get the total elongation which is proportional to web tension.

Referring now to FIG. 9, there is illustrated a diagramatic illustration of apparatus for deriving a signal in proportion to the tension of the web 21, which may be substituted for the sensor 13 of FIG. 4. In FIG. 9, a roller 52 is provided to support a loop of the web 21, formed between rollers 53 and 54. The roller is rotatable about a shaft 55 and the shaft 55 is supported by a journal 56, which in turn is supported by a leaf spring 57 secured to the frame 58 of the printing press. The spring 57 is secured to the frame 58 at one end, and the journal 56 at its other end. An arm 59 extends horizontally from the journal 56 and the vertical position of the arm 59 movesas the spring 57 is flexed in response to changes in the tension of the web 21. An increase in tension moves the arm 59 downwardly as shown in FIG. 9, and a decrease in tension allows the spring 57 to move the arm 59 upwardly. A conventional induction type sensor 60, employing, for example, a differential transformer with a movable slug disposed within the transformer coils, derives an electrical signal as a function of the movement of the arm 59. Although the apparatus of FIG. 9 has been described as if FIG. 9 were an elevation view, it will be understood that the apparatus of FIG. 9 functions equally well in any attitude.

From the above description, it is evident that the apparatus of the present invention accomplishes the objective of permitting a rapid elimination of a registration error by controlling the tension of the web in accordance with a function derived from the existing web tension and the existing registration error, and for maintaining the web tension at a level which maintains proper registration. Since the signals derived by blocks 5 and 6 of the computers of FIGS. 3 and 5 are related to the longitudinal dimension of the web 21, rather than to time, the operation of the apparatus is independent of the velocity of the web 21.

I claim as my invention:

1. A method for maintaining registration in a multistation web-fed printing press comprising the steps of determining the registration error at a location downstream from the second station of two successive printing stations, calculating a desired value for the tension of the web extending between said two stations as a function of said registration error, and controlling the tension of the web along the entire length between said two stations in accordance with said calculated value, so that the tension of the web between'said two stations is equal to said desired calculated value.

2. The method according to claim 1, wherein said calculated value is derived in response to said registration error and to the rate of change of said registration error, relative to the longitudinal dimension of said web. 5

3. Apparatus for maintaining registration in a multistation web-fed printing press comprising misregistration sensor meansdisposed at a location downstream from the second station of two successive printing stations for producing a signal representative of the registration error, computer means connected with said misregistration sensor means and responsive thereto for producing an output signal representative of a calculated value for the desired tension of the web extending between said two stations, and actuator means connected with said computer means and responsive thereto for controlling the tension of the web between said two stations, so that the tension is equal to said desired tension.

4. Apparatus according to claim 3, wherein said computer means comprises means for deriving said output signalin response to said registration error and to the rate of change of said error, relative to the longitudinal dimension of said web.

5. Apparatus according to claim 3, wherein said computer means comprises means for deriving said output signal as the sum of signals representative of three pa rameters, one of said parameters being proportional to said registration error, a second of said parameters being proportional to the rate of change in said registration error, relative to the longitudinal dimension of said web, and a third of said parameters being proportional to the sum of a plurality of said registration errors determined for equally spaced locations along the longitudinal dimension of said web.

6. Apparatus according to claim 5, including means for establishing predetermined limits on the value of said third parameter.

7. Apparatus according to claim 3, including means for limiting said output signal in accordance with predetermined limits on the maximum and minimum values for the tension of said web.

8. Apparatus according to claim 3, including manually operable means for producing a control signal representative of the nominal tension of said web, and means connecting said computer means with said manually operable means, whereby the output signal produced by said computer means is responsive to said control signal.

9. A method for maintaining registration in a multistation web-fed printing press comprising the steps of determining the registration error at a location downstream from the second station of two successive printing stations, calculating a value for the tension of the web extending between said two stations as a function of said registration error, controlling the tension of the web between said two stations in accordance with said calculated value, measuring the actual tension of the web between said two stations, comparing the measured tension with said calculated value, and changing the tension of the web between said stations so that said measured tension equals said calculated value.

10. Apparatus for maintaining registration in a multistation web-fed printing press comprising misregistration sensor means disposed at a location downstream from the second station of two successive printing sta- I tions for producing a signal representative of the registration error, computer means connected with said misregistration sensor means and responsive thereto for producing an output signal representative of a calculated value for the tension of the web extending between said two stations, actuator means connected with said computer means and responsive thereto for controlling the tension of the web between said two stations, tension responsive sensor means for producing a signal representative of the tension of said web between said two stations, and switch means for selectively connecting said computer means with said tension responsive sensor. I

1 1. Apparatus for maintaining registration in a multistation web-fed printing press comprising misregistration sensor means disposed at a location downstream from the second station of two successive printing stations for producing a signal representative of the registration error, computer means connected with said misregistration sensor means and responsive thereto for producing an output signal representative of a calculated value for the tension of the web extending between said two stations, actuator means connected with said computer means and responsive thereto for controlling the tension of the web between said two stations, tension responsive sensor means for producing a signal representative of the actual tension of said web between said two stations, and means for controlling said actuator means in response to the difference between said output signal and the signal produced by said tension responsive sensor means.

III 

1. A method for maintaining registration in a multistation webfed printing press comprising the steps of determining the registration error at a location downstream from the second station of two successive printing stations, calculating a desired value for the tension of the web extending between said two stations as a function of said registration error, and controlling the tension of the web along the entire length between said two stations in accordance with said calculated value, so that the tension of the web between said two stations is equal to said desired calculated value.
 2. The method according to claim 1, wherein said calculated value is derived in response to said registration error and to the rate of change of said registration error, relative to the longitudinal dimension of said web.
 3. Apparatus for maintaining registration in a multistation web-fed printing press comprising misregistration sensor means disposed at a location downstream from the second station of two successive printing stations for producing a signal representative of the registration error, computer means connected with said misregistration sensor means and responsive thereto for producing an output signal representative of a calculated value for the desired tension of the web extending between said two stations, and actuator means connected with said computer means and responsive thereto for controlling the tension of the web between said two stations, so that the tension is equal to said desired tension.
 4. Apparatus according to claim 3, wherein said computer means comprises means for deriving said output signal in response to said registration error and to the rate of change of said error, relative to the longitudinal dimenSion of said web.
 5. Apparatus according to claim 3, wherein said computer means comprises means for deriving said output signal as the sum of signals representative of three parameters, one of said parameters being proportional to said registration error, a second of said parameters being proportional to the rate of change in said registration error, relative to the longitudinal dimension of said web, and a third of said parameters being proportional to the sum of a plurality of said registration errors determined for equally spaced locations along the longitudinal dimension of said web.
 6. Apparatus according to claim 5, including means for establishing predetermined limits on the value of said third parameter.
 7. Apparatus according to claim 3, including means for limiting said output signal in accordance with predetermined limits on the maximum and minimum values for the tension of said web.
 8. Apparatus according to claim 3, including manually operable means for producing a control signal representative of the nominal tension of said web, and means connecting said computer means with said manually operable means, whereby the output signal produced by said computer means is responsive to said control signal.
 9. A method for maintaining registration in a multi-station web-fed printing press comprising the steps of determining the registration error at a location downstream from the second station of two successive printing stations, calculating a value for the tension of the web extending between said two stations as a function of said registration error, controlling the tension of the web between said two stations in accordance with said calculated value, measuring the actual tension of the web between said two stations, comparing the measured tension with said calculated value, and changing the tension of the web between said stations so that said measured tension equals said calculated value.
 10. Apparatus for maintaining registration in a multi-station web-fed printing press comprising misregistration sensor means disposed at a location downstream from the second station of two successive printing stations for producing a signal representative of the registration error, computer means connected with said misregistration sensor means and responsive thereto for producing an output signal representative of a calculated value for the tension of the web extending between said two stations, actuator means connected with said computer means and responsive thereto for controlling the tension of the web between said two stations, tension responsive sensor means for producing a signal representative of the tension of said web between said two stations, and switch means for selectively connecting said computer means with said tension responsive sensor.
 11. Apparatus for maintaining registration in a multi-station web-fed printing press comprising misregistration sensor means disposed at a location downstream from the second station of two successive printing stations for producing a signal representative of the registration error, computer means connected with said misregistration sensor means and responsive thereto for producing an output signal representative of a calculated value for the tension of the web extending between said two stations, actuator means connected with said computer means and responsive thereto for controlling the tension of the web between said two stations, tension responsive sensor means for producing a signal representative of the actual tension of said web between said two stations, and means for controlling said actuator means in response to the difference between said output signal and the signal produced by said tension responsive sensor means. 